1. Field of the Invention
The present invention relates to luminescent solar energy concentrator devices and more particularly, to configurational improvements in a multiple-dye containing luminescent solar concentrator (LSC).
2. Description of the Prior Art
Ground level solar energy radiation is so dilute that devices which concentrate sunlight are utilized before converting it to electrical or thermal energy. These devices typically use geometric optics in the form of mirrors and lenses. In doing so they incur the problems of cost and the requirement of tracking. Optics with high concentration capabilities need to track or be re-directed towards the sun on an hourly basis, while even moderate gain devices require adjustments for seasonal changes. Design tolerances and good weathering capability, as well as the need for tracking, make a geometrical optics concentrator an expensive alternative.
A promising new type of solar collector, the Luminescent Solar Concentrator (LSC), has recently been proposed based on light-pipe trapping of molecular fluorescence. Conceptually, the concentrator process proceeds as follows: the solar flux passes into a transparent substrate such as a flat plate which has an index of refraction greater than that of air. The solar photons are absorbed and subsequently randomly reemitted in all directions by fluorescence of an efficient fluorescent dye. Snell's law dictates that a large fraction, typically 75%, of this reemission strikes the surface of the substrate with an angle of incidence greater than the critical angle, so that this fraction of the light is then trapped in the substrate by internal reflection until successive reflection carries it to the edge of the plate where it enters an absorber placed at the edge of the plate. If light enters the face of area A.sub.f and leaves via an edge of area A.sub.e, the net geometric gain G.sub.geom is: EQU G.sub.geom =A.sub.f /A.sub.e (I)
Of the proposed solar-energy technologies, photovoltaic conversion offers the unique advantages of direct conversion of photons to electrical energy combined with simplicity, low maintenance, and high redundancy. Important problems related to the improvement of photovoltaic conversion efficiency are (a) how to maximize the fraction of the solar spectrum collected and (b) how to improve the methods for coupling solar photons to the photovoltaic junction. The LSC can potentially aid in the solution of both (a) and (b). Important advantages of the LSC collector are: it does not have to be steered to track the sun; it is nearly as efficient in diffuse as in direct sunlight; it reduces heat dissipation in the edge-mounted solar cells, since the excess energy of the short-wavelength photons is dissipated over the entire area of the LSC.
A series of such concentrators, each containing a different dye and attached to a different type of solar cell, can be stacked in such a way that the solar spectrum could be sorted and routed to a variety of photovoltaic devices. It also has been shown that if a number of properly selected dyes were placed within the same substrate, that not only did the absorption band increase but that radiative and non-radiative transfer of excitations between the dyes could take place, greatly increasing the overall collection efficiency. Each dye will absorb the solar flux within its absorption band. The emission of this dye is absorbed by a different dye, forming the so-called photon cascade. The device has been designated as the multiple dye Planar Solar Collector (PSC.sub.m), which is an extension of the single dye PSC.sub.s.
The principle advantages of the LSC or PSC is its low initial cost and indifference to solar angle of incidence. It should be noted though that while the LSC is a high gain collector it is also a lossy collector. A fraction of the photon's energy is dissipated in the cascade process, and this fraction would be waste heat for photothermal collection. Because of this the primary application will be in photovoltaic conversion. Efficient photovoltaic cells (PVC) are costly, and hence will benefit most dramatically from the high geometric gains possible in an LSC; in fact, their efficiency increases with such high flux levels. Further, waste energy usually dissipated in the PVC is released in the LSC matrix, reducing the cell's operating temperature.
The present invention relates to geometric improvements designed to provide increased and uniform output of light at the edge of luminescent solar concentrators.